In the stretch-blow molding machine known from DE 10 2008 012 757 A (rotary machine), an electronic control unit in the rotating part is associated to each blow molding station and controls the stretching rod movement as well as the blow pressure and valve control to be able to quickly adapt the control for the production process when the product is changed. The control units in the rotating part are linked via a bus system and connected to a master control device outside the rotating part. Each blow molding station can have a timer to perform angle-independent control. The positions the stretching rod passes in the production process can be stored in recipes for different types of containers. The stretching speed can be freely parameterized independent of the production speed. Each control unit can comprise a servo frequency converter and does not only control the electric stretching, but ideally also all functions of the blow molding station. A cycle is initiated by a starting signal and is then timed-controlled. In each control unit, a computer program is so to speak processed. A time-dependent stretching progress can be stored in the control unit. The electric servomotor for the stretching rod control is a rotating frequency converter motor, where frequency conversion can also take care of the control of the valves. The electric servomotor drives a stretching slide supporting the stretching rod over a belt. The rotating part is supplied with the operating media by at least one rotary transmission leadthrough between the rotating and the stationary part.
Furthermore, stretch-blow molding machines in which the movement of the stretching rod is controlled by cam control combined with at least one pneumatic cylinder in each blow molding station are known in practice. It is furthermore known from practice to combine the electric servomotor with a pneumatic cylinder also in electric stretching.
From EP 1 066 149 B, a stretch-blow molding machine whose blow molding stations are disposed stationarily is known. The stretching rod drive system works with an electric servomotor which drives the stretching slide supporting the stretching rod via a sprocket belt and causes the movement of the stretching rod in a path-controlled or force-controlled manner over time. One electronic control unit is associated to each blow molding station, which optionally comprises several blow molds, where the control unit controls the movement of the stretching rod and the application of blow pressure in closed control loops. According to the method, suited curves can be selected from different stored curves among the stretching rod path over time, the stretching rod force over time, or the blow nozzle pressure over time, and then processed. In the path-controlled stretching rod movement, at most one change of speed is performed. Before the start of production, first and second calibrations of the stretching rod control that will be accomplished later in the production process are carried out slowly, and their results are stored. In the first calibration, the stroke distance of the stretching rod up to a distance from the mold cavity bottom corresponding to the thickness of the bottle bottom is determined, i.e. optionally including an after-pressure path for the preform bottom. In the second calibration, the stroke distance of the stretching rod to the preform bottom of the cold preform minus a stroke increment for e.g. temperature-related deformations of hot preforms are determined. Based on the calibrations, the control loops generate for example time-related stroke curves for the actual production process. Similarly, time-related pressure curves for a preblow and a final blow phase can also be generated.
In a known method, for example in a stretch-blow molding machine with a combination of cam control and a pneumatic drive in the stretching rod drive system, the course of movement of the stretching rod is carried out as a first process step before the preblow phase. This first process step, however, can only be carried out when the free end of the stretching rod contacts the preform bottom. The preblow phase, however, cannot be started before the blow nozzle has been placed onto the preform mouth in a sealing manner. With the cam control, the stretching rod cannot have performed a relatively short stroke before the blow nozzle has been placed in a sealing manner. Only afterwards, the stretching rod can perform the further, possibly clearly longer stroke until it contacts the preform bottom, which involves a considerable idle time. Idle time means an essential non-utilized proportion of the process angle in the production process and is the longer, the longer the preform is. A longer stroke of the stretching rod already before the sealing time of the blow nozzle is not possible, as otherwise the stretching process will start too early in case of a short preform.